The present invention generally relates to methods for modifying the cross-sectional area of a hole. More particularly, this invention relates to a coating process that can be controlled to selectively resize a hole, a nonlimiting example being a premix fuel supply hole of a fuel nozzle assembly of a gas turbine.
In gas turbines, a fuel nozzle typically comprises a subassembly of generally concentric tubes defining a central passage for supplying diffusion fuel gas and a pair of concentric passages for supplying premix fuel gas. Spaced from and surrounding the subassembly is an inlet flow conditioner for directing and confining a flow of inlet air past a plurality of circumferentially spaced vanes carried by the subassembly. The vanes are in communication with the concentric fuel gas supply passages. Particularly, the vanes include outer and inner premix fuel supply holes for supplying gas from the respective passages for mixing with the inlet air. The gas fuel mixture is swirled by the vanes downstream of the premix fuel supply holes for subsequent combustion.
FIGS. 1, 2 and 3 represent a non-limiting example of a conventional fuel nozzle assembly 10 for a land-based gas turbine in accordance with an aspect of the invention. Generally, the fuel nozzle assembly 10 includes a subassembly 28 and a surrounding air inlet conditioner 30. The subassembly 28 includes a central tube 12 and a pair of concentric tubes 14 and 16 defining therebetween discrete annular fuel passages 18 and 20. The central tube 12 supplies diffusion gas to a combustion zone (not shown) located downstream of the fuel nozzle assembly 10. The subassembly 28 further includes a plurality of vanes 22 that are shown in FIG. 2 as circumferentially spaced from each other around the outer tube 16. The vanes 22 include outer premix fuel supply holes 24 supplied with gaseous fuel from the passage 20 and a plurality of inner premix fuel supply holes 26 supplied with gaseous fuel from the passage 18. As best seen in FIGS. 2 and 3, each vane 22 has a pair of outer and inner plenums 32 and 34, respectively, confined between opposite side walls 36 and 38 of the vane 22. The holes 24 and 26 are fluidically connected with the passages 20 and 18 through the outer and inner plenums 32 and 34, respectively.
As represented in FIG. 2, the outer premix fuel supply holes 24 include a pair of radially spaced premix fuel supply holes 24 through one wall 36 of the vane 22 and a single premix fuel supply hole 24 through the opposite side wall 38 of the vane 22. Downstream portions 40 of the vanes 22 are represented in FIG. 2 as twisted to impart a swirl to the flow of premixed air and gaseous fuel flowing between the subassembly 28 and the inlet flow conditioner 30, the gaseous fuel being supplied to the air stream via the outer and inner premix fuel supply holes 24 and 26, respectively.
The gas fuel composition and Wobbe Index (an indicator of the interchangeability of fuel gases) at site locations determine the fuel gas nozzle exit velocity requirement, which in turn is dependent upon the premix fuel supply hole size. Where the premix fuel supply holes 24 are too large for a given gas composition and Wobbe Index, nozzle dynamics become a concern. This oversized orifice may be the result of wear or a mistake in original orifice dimension. Typically, as in the case of the fuel nozzle assembly 10, one or more of the premix fuel supply holes 24 being oversized may deem the part unusable for its intended purpose.
One method of repair for the fuel nozzle assembly 10 is to take it apart, replace the vane 22 with the oversized premix fuel supply holes 24, and re-assemble the nozzle assembly 10. This can be an expensive way to salvage an otherwise unusable part and can result in scrapping of the fuel nozzle assembly 10 under some situations. Another method involves inserting plugs into the premix fuel supply holes 24 and securing them to the vane 22, possibly using a braze technique. New holes are formed through at least three of the plugs to diameters less than the diameter of the original premix fuel supply holes 24. Thus, the original premix fuel supply holes 24 are resized to provide smaller holes with consequent desired tuning effects. Yet another method includes welding the premix fuel supply holes 24 shut and then trying to find the original locations so they can be re-drilled to a smaller size.
All of the above solutions can be expensive and time consuming, among other individual disadvantages. For example, solutions that involve techniques such as welding can be difficult to perform without damaging the vane 22 and braze joints that may have been used to fabricate the assembly 10.
In view of the above, it can be appreciated that there is a need for an improved method of resizing premix fuel supply holes of fuel nozzle assemblies for gas turbine engines, as well as other types of holes whose cross-sectional area must be controlled. It would be particularly advantageous if such a method were capable of requiring less effort and expense than techniques such as welding, which can damage components of a complex device.